Probably neither. Are you sure that callers are getting busy or are they getting re-order (fast busy)? Although they are not incredibly frienly to gather and read, a combination of CallManager traces and Unity traces would be better.
Are caller's reporting not being able to reach Unity, but Unity reports that ports are ready to take calls? Can the issue be reproduced?
If you think it is a case where Unity is simply running out of ports to handle calls due to heavy traffic, you can try taking a look at the port busy report tool out on the 2.x tools page of AnswerMonkey. This can provide a quick look at your port usage to see if you're coming close to running out of ports durring peak periods of use or the like.
If it's a different issue (i.e. ports being burned or getting into a whacky state in TAPI land or whatever, Steve's suggestion of CM traces and Unity traces is the way to go.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...